It is+adj.+of sb.It is+adj.+for sb.

It is+adj.+of sb. 这里的形容词是形容人的, 你可以假设人是---的,若通顺就用of
eg: It is kind of you to help me. (you are kind.)
It is friendly of them to lend money to me. (they are friendly.)
It is+adj.+for sb.
这里的形容词是修饰后面的物或动作的.与人没关系,你假设人是---的,不通顺就用for .
It is hard for him to carry the heavy box. (He is hard.)

看那个形容词.
当那个形容词是形容人的品格,人的特征时,用of;例:It's very kind of you to help me.
当那个形容词是形容做的事的特点时,用for.例:It's very difficult for you to finish much
homework in one hour.